For the purposes of the present disclosure, the term “in-service” shall be understood to refer to an optical link in which at least one channel is carrying (or is available to carry) data traffic. Similarly, “in-service OTDR”, and “in-service Raman” and the like shall be understood to refer to OTDR measurements and Raman amplification operation pertaining to an “in-service” optical fiber link.
FIGS. 1A-1D illustrate representative Wavelength Division Multiplexed (WDM) or Dense Wavelength Division Multiplexed (DWDM) optical systems 2 known in the art. In all four examples, the system 2 comprises an optical fiber link 3 having a known length which extends between a set of transmitters 6 at a transmitting end of the system 2, and a corresponding set of receivers 8 at a receiving end of the system. Typically, the optical link 3 will include two or more optical fiber spans 4, interconnected by discrete optical devices 5 such as, for example, Erbium Doped Fiber Amplifiers EDFAs and optical switches. In the illustrated arrangement, optical MUX and DEMUX devices, 10, 12 are provided to combine optical channel signals from each transmitter into a WDM signal for transmission through the optical fiber span, and for supplying each wavelength channel from the WDM signal to a respective receiver. However, it will be appreciated that other suitable MUX/DEMUX arrangements may be used if desired. For the purposes of the present invention, each fiber span 4 can be any suitable type and length; each transmitter may be tuned to any suitable wavelength channel and may use any suitable modulation technique to transmit data through the fiber span; and each receiver may use any suitable detection (i.e. direct or coherent detection) and decoding techniques.
Optical Time Domain Reflectometry (OTDR) is a well-known technique which can be used to obtain an impulse response of an optical fiber span 4, and extract useful information regarding optical properties of the fiber link 3. For example, OTDR has been successfully used to obtain information about fiber attenuation profile (i.e. loss vs. distance), and point losses and/or reflections due to physical problems with the fiber (such as a “pinched” fiber). Degradation or impairment of fiber properties over time may be monitored by saving a reference OTDR measurement, and comparing it with a new OTDR measurement made at a later time.
In the examples of FIGS. 1A-1D, OTDR is implemented by means of an OTDR subsystem 14 connected to the optical fiber link 3 at a suitable location. In the illustrated examples, only one OTDR subsystem 14 is shown. However, in practice, any suitable number of OTDR subsystems 14 may be employed, and these may be connected to the optical fiber link 3 at any suitable location(s). In the examples of FIGS. 1A and 1B, the OTDR sub-system 14 is connected to the fiber link 3 such that OTDR pulses are launched into the fiber span 4 and propagate in the same direction as traffic launched from a transmitter 6. This is referred to as co-propagating OTDR. In the examples of FIGS. 1C and 1D, the OTDR sub-system 14 is connected to the fiber link 3 such that OTDR pulses are launched into the fiber span 4 and propagate in the opposite direction as traffic launched from a transmitter 6. This is referred to as counter-propagating OTDR.
An advantage of OTDR is that it permits measurement of the optical characteristics of an optical fiber link installed in a network. Typically, OTDR is used to evaluate the performance of unused (i.e. “dark”) fibers, which have been installed, in the network but which are not carrying any optical channel signals. This provides an effective means of determining whether or not an installed optical fiber link is ready to support optical channel traffic when needed. However, in some cases it is also possible to perform OTDR measurements of “in-service” fibers by selecting a wavelength for the OTDR measurements that will not interfere with optical channel signals within the fiber.
Meanwhile, the increasing demand for bandwidth and signal reach is driving the development of technologies capable of transmitting more and more bits per second in the DWDM (dense wavelength division multiplexing) spectrum. Supporting such demands requires better Optical Signal-to-Noise ratio (OSNR) for each traffic signal. A commonly used technique to improve OSNR is to deploy Raman amplifiers employing co- or counter propagating pump lasers. Raman amplifiers work by taking the advantage of nonlinear Stimulated Raman Scattering (SRS) phenomena in the fiber, and the interaction between Raman pump laser light and the optical channel signals propagating in the fiber.
Typically, Raman amplification is implemented by means of a Raman module 16 or card, that includes a pump laser 18 and a coupler 20 for optically coupling pump light from the pump laser 18 into an optical fiber span 4 at a desired location. As the pump light propagates through the link 3, energy is coupled from the pump light into any optical signals having a wavelength lying within a Raman gain region, thereby amplifying any such optical signals. In the examples of FIGS. 1A and 1D, the Raman module 16 is connected to the fiber link 3 such that the pump light propagate in the same direction as traffic launched from a transmitter 6. This is referred to as co-propagating Raman. In the examples of FIGS. 1B and 1C, the Raman module 16 is connected to the fiber link 3 such that the pump light propagates in the opposite direction as traffic launched from a transmitter 6. This is referred to as counter-propagating Raman.
In practical optical transmission systems, any suitable number of Raman modules 14 may be provided, and may be connected to the link 3 at any suitable location. For example, Raman modules may be provided at both the transmitter and receiver ends of the link 3. If desired, one or more Raman modules may be provided at a discrete optical device 5 so as to inject pump light into a desired fiber span 4. In systems having more than one Raman module 14, the respective Raman pump lasers 18 may be tuned to a common wavelength, or to respective different wavelengths, as desired. In some cases, a Raman module may include more than one pump laser 18, and these lasers may be tuned to the same, or different wavelengths, as desired.
In FIGS. 1A-1D, the length of a fiber span 4 (L) is measured from the point at which the OTDR sub-system 14 is connected to the fiber. Thus, in FIGS. 1A and 1B, the origin (z=0) is located at the transmitter end of a span 4, and distances are measured toward the receiver end of the link 3. Conversely, in FIGS. 1C and 1D, the origin (z=0) is located at the discrete optical device 5, and distances are measured toward the transmitter end of the link 3. This nomenclature is relevant to the present technique because OTDR measures distance from the point of injection of the OTDR pulses.
Because Raman amplification utilizes the fiber itself as the gain media, the optical characteristics of the installed fiber link must be well defined before turning the pump laser(s) on. This is very important both in terms of safety and performance, as pump laser power levels can be high enough to cause damage due to point losses such as poor splices or dirty connectors. Conventional OTDR methods can be used to accomplish the required characterization of the installed optical fiber link. Typically, a so-called “short trace” is used to characterise portions of a fiber link where Raman gain is expected to be high, which corresponds to the portion of the fiber nearest to the Raman module. If the short trace identifies a problem with the fiber, such as a high loss, the fiber link is disqualified and a service technician or management system may be notified. A long trace is typically used to detect faults along the entire length of the fiber link, and so may be used to locate a pinch or cut in the fiber span. The short and long OTDR traces are commonly used together to obtain a “snapshot” of the optical link characteristics.
However, when the Raman pump lasers are turned ON, OTDR traces are adversely affected by nonlinear Raman scattering from the pump light. Because the intensities of both the Raman scattering and the OTDR return signal are affected by the properties of the optical fiber, there is no reliable method of analysing OTDR trace information to distinguish degradations in Raman gain from degradations in fiber performance. As a result, the conventional use of OTDR for analysing optical fiber characteristics is limited to situations in which the Raman pump lasers are turned OFF, which implies that the transmitters 6 are not transmitting optical signal traffic through the fiber link 3.
As noted above, conventional OTDR trace information is obtained when Raman amplification is OFF, i.e., no Raman gain and therefore no traffic (which implies that the optical link 3 is out of service). The two conventional applications of OTDR comprise: short trace, to investigate if the fiber qualities for Raman amplification with regard to safety and performance concerns; and long trace, in fault scenarios such as fiber cut when an automatic fiber cut location analysis is performed. In both of these applications, OTDR trace information is obtained only when the optical link is out of-service.
Consequently, network elements typically use a telemetry channel to estimate the overall gain experienced by wavelength channels/signals when the link is in-service. This technique provides a blind estimation of the gain due to the presence of Raman pump light. As a result any degradation in provided Raman gain cannot be simply analyzed to be due to the real degradation in Raman gain or due to non-Raman related issues.
It would be desirable to provide techniques for analysing in-service fiber link characteristics using real-time OTDR trace information in the presence of Raman amplification. In this respect, “real-time” should be understood to refer to information that is obtained from measurements with minimum processing delay, so that the measurements and the information obtained therefrom, provide a high-fidelity representation of the current state and/or performance of the optical fiber link.